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2009-07-22 | Searching for variable stars in Galactic Open Clusters | A long-term project, aiming at systematic search for variable stars in
Galactic Open Clusters, was started at the Geneva Observatory in 2002. We have
been observing regularly a sample of twenty-seven Galactic Open Clusters in the
U, B, V Geneva filters. The goal is to identify and to study their variable
stars, as well as the connection between the variable stars in a cluster and
the cluster properties. We present the status of this work in progress, and
show preliminary results for one of these clusters, IC 4651. | 0907.3834v1 |
2009-07-23 | Variability morphologies in the color-magnitude diagram. Searching for secular variability | This work is part of an effort to detect secular variable objects in large
scale surveys by analysing their path in color-magnitude diagrams. To this aim,
we first present the variability morphologies in the V/V-I diagram of several
types of variable stars. They comprise both periodic and non periodic variable
stars from the Large Magellanic Cloud, such as classical Cepheids, long period
variables or Be and R Coronae Borealis stars, as well as two of the detected
secular variable stars in the Galaxy, FG Sge and V4334 Sgr. The study of the
different variability morphologies allows the identification of regions in the
color-magnitude diagram where those secular variable stars could be detected.
We also estimate the number of such secular variable stars expected in the
Large Magellanic Cloud. | 0907.4090v1 |
2009-08-03 | Regret Bounds for Opportunistic Channel Access | We consider the task of opportunistic channel access in a primary system
composed of independent Gilbert-Elliot channels where the secondary (or
opportunistic) user does not dispose of a priori information regarding the
statistical characteristics of the system. It is shown that this problem may be
cast into the framework of model-based learning in a specific class of
Partially Observed Markov Decision Processes (POMDPs) for which we introduce an
algorithm aimed at striking an optimal tradeoff between the exploration (or
estimation) and exploitation requirements. We provide finite horizon regret
bounds for this algorithm as well as a numerical evaluation of its performance
in the single channel model as well as in the case of stochastically identical
channels. | 0908.0319v1 |
2009-08-14 | Non-Gaussian Scatter in Cluster Scaling Relations | We investigate the impact of non-Gaussian scatter in the cluster
mass-observable scaling relation on the mass and redshift distribution of
clusters detected by wide area surveys. We parameterize non-Gaussian scatter by
incorporating the third and forth moments (skewness and kurtosis) into the
distribution P(Mobs|M). We demonstrate that for low scatter mass proxies the
higher order moments do not significantly affect the observed cluster mass and
redshift distributions. However, for high scatter mass indicators it is
necessary for the survey limiting mass threshold to be less than 10^14 h^-1
Msol to prevent the skewness from having a significant impact on the observed
number counts, particularly at high redshift. We also show that an unknown
level of non-Gaussianity in the scatter is equivalent to an additional
uncertainty on the variance in P(Mobs|M) and thus may limit the constraints
that can be placed on the dark energy equation of state parameter w.
Furthermore, positive skewness flattens the mass function at the high mass end,
and so one must also account for skewness in P(Mobs|M) when using the shape of
the mass function to constrain cluster scaling-relations. | 0908.1978v1 |
2009-08-17 | A local moment approach to the degenerate Anderson impurity model | The local moment approach is extended to the orbitally-degenerate [SU(2N)]
Anderson impurity model (AIM). Single-particle dynamics are obtained over the
full range of energy scales, focussing here on particle-hole symmetry in the
strongly correlated regime where the onsite Coulomb interaction leads to
many-body Kondo physics with entangled spin and orbital degrees of freedom. The
approach captures many-body broadening of the Hubbard satellites, recovers the
correct exponential vanishing of the Kondo scale for all N, and its universal
scaling spectra are found to be in very good agreement with numerical
renormalization group (NRG) results. In particular the high-frequency
logarithmic decays of the scaling spectra, obtained here in closed form for
arbitrary N, coincide essentially perfectly with available numerics from the
NRG. A particular case of an anisotropic Coulomb interaction, in which the
model represents a system of N `capacitively-coupled' SU(2) AIMs, is also
discussed. Here the model is generally characterised by two low-energy scales,
the crossover between which is seen directly in its dynamics. | 0908.2245v1 |
2009-12-08 | Evidence of superconductivity on the border of quasi-2D ferromagnetism in Ca2RuO4 at high pressure | The layered perovskite Ca2RuO4 is a spin-one Mott insulator at ambient
pressure and exhibits metallic ferromagnetism at least up to ~ 80 kbar with a
maximum Curie temperature of 28 K. Above ~ 90 kbar and up to 140 kbar, the
highest pressure reached, the resistivity and ac susceptibility show pronounced
downturns below ~ 0.4 K in applied magnetic fields of up to ~10 mT. This
indicates that our specimens of Ca2RuO4 are weakly superconducting on the
border of a quasi-2D ferromagnetic state. | 0912.1513v1 |
2010-02-24 | JSJ decompositions: definitions, existence, uniqueness. II. Compatibility and acylindricity | This paper and its companion arXiv:0911.3173 have been replaced by
arXiv:1602.05139.
We define the compatibility JSJ tree of a group G over a class of subgroups.
It exists whenever G is finitely presented and leads to a canonical tree (not a
deformation space) which is invariant under automorphisms. Under acylindricity
hypotheses, we prove that the (usual) JSJ deformation space and the
compatibility JSJ tree exist, and we describe their flexible subgroups. We
apply these results to finitely generated CSA groups, \Gamma-limit groups
(allowing torsion), and relatively hyperbolic groups. | 1002.4564v3 |
2010-03-05 | Magnetoresistance in nanostructures: the role of nonuniform current | We developed a method to calculate the magnetoresistance of magnetic
nanostructures. We discretize a magnetic disk in small cells and numerically
solve the Landau-Lifshitz-Gilbert (LLG) equation in order to obtain its
magnetization profile. We consider a anisotropic magnetoresistance (AMR) that
depends on the local magnetization as the main source of the magnetoresistance.
We then use it as an input to calculate the resistance and current distribution
numerically, using a relaxation method. We show how magnetoresistance
measurements can be useful to obtain information on the magnetic structure.
Additionally, we obtain non-homogeneous current distributions for different
magnetic configurations in static and dynamical regimes. | 1003.1323v2 |
2010-03-21 | Current-induced domain wall motion with adiabatic spin torque only in cylindrical nanowires | We investigate current-driven domain wall (DW) propagation in magnetic
nanowires in the framework of the modified Landau-Lifshitz-Gilbert equation
with both adiabatic and nonadiabatic spin torque (NAST) terms. Contrary to the
common opinion that NAST is indispensable for DW motion[1,2], we point out that
adiabatic spin torque (AST) only is enough for current-driven DW motion in a
cylindrical (uniaxial) nanowire. Apart from a discussion of the rigid DW motion
from the energy and angular momentum viewpoint, we also propose an experimental
scheme to measure the spin current polarization by combining both field and
current driven DW motion in a flat (biaxial) wire. | 1003.3976v1 |
2010-04-01 | Multipliers of locally compact quantum groups via Hilbert C$^*$-modules | A result of Gilbert shows that every completely bounded multiplier $f$ of the
Fourier algebra $A(G)$ arises from a pair of bounded continuous maps
$\alpha,\beta:G \rightarrow K$, where $K$ is a Hilbert space, and $f(s^{-1}t) =
(\beta(t)|\alpha(s))$ for all $s,t\in G$. We recast this in terms of
adjointable operators acting between certain Hilbert C$^*$-modules, and show
that an analogous construction works for completely bounded left multipliers of
a locally compact quantum group. We find various ways to deal with right
multipliers: one of these involves looking at the opposite quantum group, and
this leads to a proof that the (unbounded) antipode acts on the space of
completely bounded multipliers, in a way which interacts naturally with our
representation result. The dual of the universal quantum group (in the sense of
Kustermans) can be identified with a subalgebra of the completely bounded
multipliers, and we show how this fits into our framework. Finally, this
motivates a certain way to deal with two-sided multipliers. | 1004.0215v3 |
2010-04-22 | Optimal time-dependent polarized current pattern for fast domain wall propagation in nanowires: Exact solutions for biaxial and uniaxial anisotropies | One of the important issues in nanomagnetism is to lower the current needed
for a technologically useful domain wall (DW) propagation speed. Based on the
modified Landau-Lifshitz-Gilbert (LLG) equation with both Slonczewski
spin-transfer torque and the field-like torque, we derive the optimal spin
current pattern for fast DW propagation along nanowires. Under such conditions,
the DW velocity in biaxial wires can be enhanced as much as ten times compared
to the velocities achieved in experiments so far. Moreover, the fast variation
of spin polarization can help DW depinning. Possible experimental realizations
are discussed. | 1004.3840v1 |
2010-05-03 | Criticality of an isotropic-to-smectic transition induced by anisotropic quenched disorder | We report combined optical birefringence and neutron scattering measurements
on the liquid crystal 12CB nanoconfined in mesoporous silicon layers. This
liquid crystal exhibits strong nematic-smectic coupling responsible for a
discontinuous isotropic-to-smectic phase transition in the bulk state. Confined
in porous silicon, 12CB is subjected to strong anisotropic quenched disorder: a
short-ranged smectic state evolves out of a paranematic phase. This
transformation appears continuous, losing its bulk first order character. This
contrasts with previously reported observations on liquid crystals under
isotropic quenched disorder. In the low temperature phase, both orientational
and translational order parameters obey the same power-law. | 1005.0240v1 |
2010-05-21 | A Study on Potential of Integrating Multimodal Interaction into Musical Conducting Education | With the rapid development of computer technology, computer music has begun
to appear in the laboratory. Many potential utility of computer music is
gradually increasing. The purpose of this paper is attempted to analyze the
possibility of integrating multimodal interaction such as vision-based hand
gesture and speech interaction into musical conducting education. To achieve
this purpose, this paper is focus on discuss some related research and the
traditional musical conducting education. To do so, six musical conductors had
been interviewed to share their musical conducting learning/ teaching
experience. These interviews had been analyzed in this paper to show the
syllabus and the focus of musical conducting education for beginners. | 1005.4014v1 |
2010-05-28 | Dissipative structures in a nonlinear dynamo | This paper considers magnetic field generation by a fluid flow in a system
referred to as the Archontis dynamo: a steady nonlinear magnetohydrodynamic
(MHD) state is driven by a prescribed body force. The field and flow become
almost equal and dissipation is concentrated in cigar-like structures centred
on straight-line separatrices. Numerical scaling laws for energy and
dissipation are given that extend previous calculations to smaller
diffusivities. The symmetries of the dynamo are set out, together with their
implications for the structure of field and flow along the separatrices. The
scaling of the cigar-like dissipative regions, as the square root of the
diffusivities, is explained by approximations near the separatrices. Rigorous
results on the existence and smoothness of solutions to the steady, forced MHD
equations are given. | 1005.5259v2 |
2010-06-11 | Spherical single-roll dynamos at large magnetic Reynolds numbers | This paper concerns kinematic helical dynamos in a spherical fluid body
surrounded by an insulator. In particular, we examine their behaviour in the
regime of large magnetic Reynolds number $\Rm$, for which dynamo action is
usually concentrated upon a simple resonant stream-surface. The dynamo
eigensolutions are computed numerically for two representative single-roll
flows using a compact spherical harmonic decomposition and fourth-order
finite-differences in radius. These solutions are then compared with the growth
rates and eigenfunctions of the Gilbert and Ponty (2000) large $\Rm$ asymptotic
theory. We find good agreement between the growth rates when $\Rm>10^4$, and
between the eigenfunctions when $\Rm>10^5$. | 1006.2308v2 |
2010-06-11 | On the Queueing Behavior of Random Codes over a Gilbert-Elliot Erasure Channel | This paper considers the queueing performance of a system that transmits
coded data over a time-varying erasure channel. In our model, the queue length
and channel state together form a Markov chain that depends on the system
parameters. This gives a framework that allows a rigorous analysis of the queue
as a function of the code rate. Most prior work in this area either ignores
block-length (e.g., fluid models) or assumes error-free communication using
finite codes. This work enables one to determine when such assumptions provide
good, or bad, approximations of true behavior. Moreover, it offers a new
approach to optimize parameters and evaluate performance. This can be valuable
for delay-sensitive systems that employ short block lengths. | 1006.2403v1 |
2010-06-16 | Storing Small Photonic Cluster States in a Dephasing Environment | We consider the effects of decoherence on the entanglement of photonic
cluster states. Large photonic cluster states can be built by fusing together
smaller photonic cluster states via probabilistic fusion operations. For this
construction process it is necessary to store these smaller cluster states in
some way so as to have them available for attempted fusion operations. While in
storage the photonic cluster states may undergo dephasing. The effects of
dephasing on small, primitive cluster states is explored here with the aim of
determining how to locally rotate the qubits of the cluster state so as to lose
the least amount of entanglement due to the dephasing process. | 1006.3192v1 |
2010-08-05 | Subsystem codes with spatially local generators | We study subsystem codes whose gauge group has local generators in the 2D
geometry. It is shown that there exists a family of such codes defined on
lattices of size LxL with the number of logical qubits k and the minimum
distance d both proportional to L. The gauge group of these codes involves only
two-qubit generators of type XX and ZZ coupling nearest neighbor qubits (and
some auxiliary one-qubit generators). Our proof is not constructive as it
relies on a certain version of the Gilbert-Varshamov bound for classical codes.
Along the way we introduce and study properties of generalized Bacon-Shor codes
which might be of independent interest. Secondly, we prove that any 2D
subsystem [n,k,d] code with spatially local generators obeys upper bounds
kd=O(n) and d^2=O(n). The analogous upper bound proved recently for 2D
stabilizer codes is kd^2=O(n). Our results thus demonstrate that subsystem
codes can be more powerful than stabilizer codes under the spatial locality
constraint. | 1008.1029v1 |
2010-08-14 | Escape of resources in distributed clustering processes | In a distributed clustering algorithm introduced by Coffman, Courtois,
Gilbert and Piret \cite{coffman91}, each vertex of $\mathbb{Z}^d$ receives an
initial amount of a resource, and, at each iteration, transfers all of its
resource to the neighboring vertex which currently holds the maximum amount of
resource. In \cite{hlrnss} it was shown that, if the distribution of the
initial quantities of resource is invariant under lattice translations, then
the flow of resource at each vertex eventually stops almost surely, thus
solving a problem posed in \cite{berg91}. In this article we prove the
existence of translation-invariant initial distributions for which resources
nevertheless escape to infinity, in the sense that the the final amount of
resource at a given vertex is strictly smaller in expectation than the initial
amount. This answers a question posed in \cite{hlrnss}. | 1008.2426v1 |
2010-08-25 | The self-avoiding walk in a strip | We review the existence of the infinite length self-avoiding walk in the half
plane and its relationship to bridges. We prove that this probability measure
is also given by the limit as $\beta \rightarrow \beta_c-$ of the probability
measure on all finite length walks $\omega$ with the probability of $\omega$
proportional to $\beta_c^{|\omega|}$ where $|\omega|$ is the number of steps in
$\omega$. The self-avoiding walk in a strip $\{z : 0<\Im(z)<y\}$ is defined by
considering all self-avoiding walks $\omega$ in the strip which start at the
origin and end somewhere on the top boundary with probability proportional to
$\beta_c^{|\omega|}$ We prove that this probability measure may be obtained by
conditioning the SAW in the half plane to have a bridge at height $y$. This
observation is the basis for simulations to test conjectures on the
distribution of the endpoint of the SAW in a strip and the relationship between
the distribution of this strip SAW and SLE$_{8/3}$. | 1008.4321v2 |
2010-08-29 | Chirality control via double vortices in asymmetric Co dots | Reproducible control of the magnetic vortex state in nanomagnets is of
critical importance. We report on chirality control by manipulating the size
and/or thickness of asymmetric Co dots. Below a critical diameter and/or
thickness, chirality control is achieved by the nucleation of single vortex.
Interestingly, above these critical dimensions chirality control is realized by
the nucleation and subsequent coalescence of two vortices, resulting in a
single vortex with the opposite chirality as found in smaller dots.
Micromagnetic simulations and magnetic force microscopy highlight the role of
edge-bound halfvortices in facilitating the coalescence process. | 1008.4963v2 |
2010-09-18 | Composite excitation of Josephson phase and spin waves in Josephson junctions with ferromagnetic insulator | Coupling of Josephson-phase and spin-waves is theoretically studied in a
superconductor/ferromagnetic insulator/superconductor (S/FI/S) junction.
Electromagnetic (EM) field inside the junction and the Josephson current
coupled with spin-waves in FI are calculated by combining Maxwell and
Landau-Lifshitz-Gilbert equations. In the S/FI/S junction, it is found that the
current-voltage (I-V) characteristic shows two resonant peaks. Voltages at the
resonant peaks are obtained as a function of the normal modes of EM field,
which indicates a composite excitation of the EM field and spin-waves in the
S/FI/S junction. We also examine another type of junction, in which a
nonmagnetic insulator (I) is located at one of interfaces between S and FI. In
such a S/I/FI/S junction, three resonant peaks appear in the I-V curve, since
the Josephson-phase couples to the EM field in the I layer. | 1009.3551v3 |
2010-09-21 | On the Flow-Level Delay of a Spatial Multiplexing MIMO Wireless Channel | The MIMO wireless channel offers a rich ground for quality of service
analysis. In this work, we present a stochastic network calculus analysis of a
MIMO system, operating in spatial multiplexing mode, using moment generating
functions (MGF). We quantify the spatial multiplexing gain, achieved through
multiple antennas, for flow level quality of service (QoS) performance.
Specifically we use Gilbert-Elliot model to describe individual spatial paths
between the antenna pairs and model the whole channel by an N-State Markov
Chain, where N depends upon the degrees of freedom available in the MIMO
system. We derive probabilistic delay bounds for the system and show the impact
of increasing the number of antennas on the delay bounds under various
conditions, such as channel burstiness, signal strength and fading speed.
Further we present results for multi-hop scenarios under statistical
independence. | 1009.4091v1 |
2010-09-22 | Optimal control of magnetization dynamics in ferromagnetic heterostructures by spin--polarized currents | We study the switching-process of the magnetization in a
ferromagnetic-normal-metal multilayer system by a spin polarized electrical
current via the spin transfer torque. We use a spin drift-diffusion equation
(SDDE) and the Landau-Lifshitz-Gilbert equation (LLGE) to capture the coupled
dynamics of the spin density and the magnetization dynamic of the
heterostructure. Deriving a fully analytic solution of the stationary SDDE we
obtain an accurate, robust, and fast self-consistent model for the
spin-distribution and spin transfer torque inside general ferromagnetic/normal
metal heterostructures. Using optimal control theory we explore the switching
and back-switching process of the analyzer magnetization in a seven-layer
system. Starting from a Gaussian, we identify a unified current pulse profile
which accomplishes both processes within a specified switching time. | 1009.4296v2 |
2010-09-29 | Acoustically driven ferromagnetic resonance | Surface acoustic waves (SAW) in the GHz frequency range are exploited for the
all-elastic excitation and detection of ferromagnetic resonance (FMR) in a
ferromagnetic/ferroelectric (nickel/lithium niobate) hybrid device. We measure
the SAW magneto-transmission at room temperature as a function of frequency,
external magnetic field magnitude, and orientation. Our data are well described
by a modified Landau-Lifshitz-Gilbert approach, in which a virtual,
strain-induced tickle field drives the magnetization precession. This causes a
distinct magnetic field orientation dependence of elastically driven FMR that
we observe in both model and experiment. | 1009.5798v2 |
2010-10-05 | Effects of Strong Gravitational Lensing on Millimeter-Wave Galaxy Number Counts | We study the effects of strong lensing on the observed number counts of mm
sources using a ray tracing simulation and two number count models of unlensed
sources. We employ a quantitative treatment of maximum attainable magnification
factor depending on the physical size of the sources, also accounting for
effects of lens halo ellipticity. We calculate predicted number counts and
redshift distributions of mm galaxies including the effects of strong lensing
and compare with the recent source count measurements of the South Pole
Telescope (SPT). The predictions have large uncertainties, especially the
details of the mass distribution in lens galaxies and the finite extent of
sources, but the SPT observations are in good agreement with predictions. The
sources detected by SPT are predicted to largely consist of strongly lensed
galaxies at z>2. The typical magnifications of these sources strongly depends
on both the assumed unlensed source counts and the flux of the observed
sources. | 1010.0998v2 |
2010-10-15 | The Significance of Non-ergodicity Property of Statistical Mechanics Systems for Understanding Resting State of a Living Cell | A better grasp of the physical foundations of life is necessary before we can
understand the processes occurring inside a living cell. In his physical theory
of the cell, American physiologist Gilbert Ling introduced an important notion
of the resting state of the cell. He describes this state as an independent
stable thermodynamic state of a living substance in which it has stored all the
energy it needs to perform all kinds of biological work. This state is
characterised by lower entropy of the system than in an active state. However,
Ling's approach is primarily qualitative in terms of thermodynamics and it
needs to be characterised more specifically. To this end, we propose a new
thermodynamic approach to studying Ling's model of the living cell (Ling's
cell), the center piece of which is the non-ergodicity property which has
recently been proved for a wide range of systems in statistical mechanics [7].
These approach allowed us to develop general thermodynamic approaches to
explaining some of the well-known physiological phenomena, which can be used
for further physical analysis of these phenomena using specific physical
models. | 1010.3226v1 |
2010-11-29 | Superoperator Analysis of Entanglement in a Four-Qubit Cluster State | In this paper we utilize superoperator formalism to explore the entanglement
evolution of four-qubit cluster states in a number of decohering environments.
A four-qubit cluster state is a resource for the performance of an arbitrary
single logical qubit rotation via measurement based cluster state quantum
computation. We are specifically interested in the relationship between
entanglement evolution and the fidelity with which the arbitrary single logical
qubit rotation can be implemented in the presence of decoherence as this will
have important experimental ramifications. We also note the exhibition of
entanglement sudden death (ESD) and ask how severely its onset affects the
utilization of the cluster state as a means of implementing an arbitrary single
logical qubit rotation. | 1011.6336v2 |
2011-02-25 | Modeling Extragalactic Foregrounds and Secondaries for Unbiased Estimation of Cosmological Parameters From Primary CMB Anisotropy | Using the latest physical modeling and constrained by the most recent data,
we develop a phenomenological parameterized model of the contributions to
intensity and polarization maps at millimeter wavelengths from external
galaxies and Sunyaev-Zeldovich effects. We find such modeling to be necessary
for estimation of cosmological parameters from Planck data. For example,
ignoring the clustering of the infrared background would result in a bias in
n_s of 7 sigma. We show that the simultaneous marginalization over a full
foreground model can eliminate such biases, while increasing the statistical
uncertainty in cosmological parameters by less than 20%. The small increases in
uncertainty can be significantly reduced with the inclusion of
higher-resolution ground-based data.
The multi-frequency analysis we employ involves modeling 46 total power
spectra and marginalization over 17 foreground parameters. We show that we can
also reduce the data to a best estimate of the CMB power spectra, and just two
principal components (with constrained amplitudes) describing residual
foreground contamination. | 1102.5195v1 |
2011-03-21 | Delay Constrained Throughput Analysis of a Correlated MIMO Wireless Channel | The maximum traffic arrival rate at the network for a given delay guarantee
(delay constrained throughput) has been well studied for wired channels.
However, few results are available for wireless channels, especially when
multiple antennas are employed at the transmitter and receiver. In this work,
we analyze the network delay constrained throughput of a multiple input
multiple output (MIMO) wireless channel with time-varying spatial correlation.
The MIMO channel is modeled via its virtual representation, where the
individual spatial paths between the antenna pairs are Gilbert-Elliot channels.
The whole system is then described by a K-State Markov chain, where K depends
upon the degree of freedom (DOF) of the channel. We prove that the DOF based
modeling is indeed accurate. Furthermore, we study the impact of the delay
requirements at the network layer, violation probability and the number of
antennas on the throughput under different fading speeds and signal strength. | 1103.4016v1 |
2011-03-29 | Current induced rotational torques in the skyrmion lattice phase of chiral magnets | In chiral magnets without inversion symmetry, the magnetic structure can form
a lattice of magnetic whirl lines, a two-dimensional skyrmion lattice,
stabilized by spin-orbit interactions in a small range of temperatures and
magnetic fields. The twist of the magnetization within this phase gives rise to
an efficient coupling of macroscopic magnetic domains to spin currents. We
analyze the resulting spin-transfer effects, and, in particular, focus on the
current induced rotation of the magnetic texture by an angle. Such a rotation
can arise from macroscopic temperature gradients in the system as has recently
been shown experimentally and theoretically. Here we investigate an alternative
mechanism, where small distortions of the skyrmion lattice and the transfer of
angular momentum to the underlying atomic lattice play the key role. We employ
the Landau-Lifshitz-Gilbert equation and adapt the Thiele method to derive an
effective equation of motion for the rotational degree of freedom. We discuss
the dependence of the rotation angle on the orientation of the applied magnetic
field and the distance to the phase transition. | 1103.5548v2 |
2011-04-07 | Finite-size effects on the magnetoelectric response of field-driven ferroelectric/ferromagnetic chains | We study theoretically the coupled multiferroic dynamics of one-dimensional
ferroelectric/ferromagnet chains driven by harmonic magnetic and electric
fields as a function of the chain length. A linear magnetoelectric coupling is
dominated by the spin-polarized screening charge at the interface. We performed
Monte-Carlo simulations and calculations based on the coupled
Landau-Lifshitz-Gilbert and Landau-Khalatnikov equations showing that the net
magnetization and the total polarization of thin heterostructures, i.e. with up
to ten ferroelectric and ferromagnetic sites counted from the interface, can be
completely reversed by external electric and magnetic fields, respectively.
However, for larger system solely a certain magnetoelectrical control can be
achieved. | 1104.1269v3 |
2011-04-12 | The Effect of Disorder in Superfluid Double Layer Graphene | We investigate the superfluid properties of disordered double layer graphene
systems using the non-equilibrium Green's function (NEGF) formalism. The
complexity of such a structure makes it imperative to study the effects of
lattice vacancies which will inevitably arise during fabrication. We present
and compare room temperature performance characteristics for both ideal and
disordered bilayer graphene systems in an effort to illustrate the behavior of
a Bose-Einstein Condensate in the presence of lattice defects under
non-equilibrium conditions. We find that lattice vacancies spread throughout
the top layer past the coherence length have a reduced effect compared to the
ideal case. However, vacancies concentrated near the metal contacts within the
coherence length significantly alter the interlayer superfluid transport
properties. | 1104.2343v1 |
2011-05-30 | Orthogonal Matching Pursuit: A Brownian Motion Analysis | A well-known analysis of Tropp and Gilbert shows that orthogonal matching
pursuit (OMP) can recover a k-sparse n-dimensional real vector from 4 k log(n)
noise-free linear measurements obtained through a random Gaussian measurement
matrix with a probability that approaches one as n approaches infinity. This
work strengthens this result by showing that a lower number of measurements, 2
k log(n - k), is in fact sufficient for asymptotic recovery. More generally,
when the sparsity level satisfies kmin <= k <= kmax but is unknown, 2 kmax
log(n - kmin) measurements is sufficient. Furthermore, this number of
measurements is also sufficient for detection of the sparsity pattern (support)
of the vector with measurement errors provided the signal-to-noise ratio (SNR)
scales to infinity. The scaling 2 k log(n - k) exactly matches the number of
measurements required by the more complex lasso method for signal recovery with
a similar SNR scaling. | 1105.5853v1 |
2011-06-07 | Magneto-optical Kerr Effect Studies of Square Artificial Spin Ice | We report a magneto-optical Kerr effect study of the collective magnetic
response of artificial square spin ice, a lithographically-defined array of
single-domain ferromagnetic islands. We find that the anisotropic inter-island
interactions lead to a non-monotonic angular dependence of the array coercive
field. Comparisons with micromagnetic simulations indicate that the two
perpendicular sublattices exhibit distinct responses to island edge roughness,
which clearly influence the magnetization reversal process. Furthermore, such
comparisons demonstrate that disorder associated with roughness in the island
edges plays a hitherto unrecognized but essential role in the collective
behavior of these systems. | 1106.1394v2 |
2011-06-15 | Dissipative hydrodynamic equation of a ferromagnetic Bose-Einstein condensate: Analogy to magnetization dynamics in conducting ferromagnets | The hydrodynamic equation of a spinor Bose-Einstein condensate (BEC) gives a
simple description of spin dynamics in the condensate. We introduce the
hydrodynamic equation of a ferromagnetic BEC with dissipation originating from
the energy dissipation of the condensate. The dissipative hydrodynamic equation
has the same form as an extended Landau-Lifshitz-Gilbert (LLG) equation, which
describes the magnetization dynamics of ferromagnets interacting with
spin-polarized currents. Employing the dissipative hydrodynamic equation, we
demonstrate the magnetic domain pattern dynamics of a ferromagnetic BEC in the
presence and absence of a current of particles, and discuss the effects of the
current on domain pattern formation. We also discuss the characteristic lengths
of domain patterns that have domain walls with and without finite
magnetization. | 1106.2876v4 |
2011-08-01 | Noise Sensitivity in Continuum Percolation | We prove that the Poisson Boolean model, also known as the Gilbert disc
model, is noise sensitive at criticality. This is the first such result for a
Continuum Percolation model, and the first for which the critical probability
p_c \ne 1/2. Our proof uses a version of the Benjamini-Kalai-Schramm Theorem
for biased product measures. A quantitative version of this result was recently
proved by Keller and Kindler. We give a simple deduction of the
non-quantitative result from the unbiased version. We also develop a quite
general method of approximating Continuum Percolation models by discrete models
with p_c bounded away from zero; this method is based on an extremal result on
non-uniform hypergraphs. | 1108.0310v2 |
2011-08-23 | Vortex Lines in Topological Insulator-Superconductor Heterostructures | 3D topological insulator/s-wave superconductor heterostructures have been
predicted as candidate systems for the observation of Majorana fermions in the
presence of superconducting vortices. In these systems, Majorana fermions are
expected to form at the interface between the topological insulator and the
superconductor while the bulk plays no role. Yet the bulk of a 3D topological
insulator penetrated by a magnetic flux is not inert and can gap the surface
vortex modes destroying their Majorana nature. In this work, we demonstrate the
circumstances under which only the surface physics is important and when the
bulk physics plays an important role in the location and energy of the Majorana
modes. | 1108.4711v1 |
2011-08-28 | Low-complexity quantum codes designed via codeword-stabilized framework | We consider design of the quantum stabilizer codes via a two-step,
low-complexity approach based on the framework of codeword-stabilized (CWS)
codes. In this framework, each quantum CWS code can be specified by a graph and
a binary code. For codes that can be obtained from a given graph, we give
several upper bounds on the distance of a generic (additive or non-additive)
CWS code, and the lower Gilbert-Varshamov bound for the existence of additive
CWS codes. We also consider additive cyclic CWS codes and show that these codes
correspond to a previously unexplored class of single-generator cyclic
stabilizer codes. We present several families of simple stabilizer codes with
relatively good parameters. | 1108.5490v1 |
2011-09-16 | Parallel Sparse Matrix-Matrix Multiplication and Indexing: Implementation and Experiments | Generalized sparse matrix-matrix multiplication (or SpGEMM) is a key
primitive for many high performance graph algorithms as well as for some linear
solvers, such as algebraic multigrid. Here we show that SpGEMM also yields
efficient algorithms for general sparse-matrix indexing in distributed memory,
provided that the underlying SpGEMM implementation is sufficiently flexible and
scalable. We demonstrate that our parallel SpGEMM methods, which use
two-dimensional block data distributions with serial hypersparse kernels, are
indeed highly flexible, scalable, and memory-efficient in the general case.
This algorithm is the first to yield increasing speedup on an unbounded number
of processors; our experiments show scaling up to thousands of processors in a
variety of test scenarios. | 1109.3739v2 |
2011-10-06 | A new class of codes for Boolean masking of cryptographic computations | We introduce a new class of rate one-half binary codes: {\bf complementary
information set codes.} A binary linear code of length $2n$ and dimension $n$
is called a complementary information set code (CIS code for short) if it has
two disjoint information sets. This class of codes contains self-dual codes as
a subclass. It is connected to graph correlation immune Boolean functions of
use in the security of hardware implementations of cryptographic primitives.
Such codes permit to improve the cost of masking cryptographic algorithms
against side channel attacks. In this paper we investigate this new class of
codes: we give optimal or best known CIS codes of length $<132.$ We derive
general constructions based on cyclic codes and on double circulant codes. We
derive a Varshamov-Gilbert bound for long CIS codes, and show that they can all
be classified in small lengths $\le 12$ by the building up construction. Some
nonlinear permutations are constructed by using $\Z_4$-codes, based on the
notion of dual distance of an unrestricted code. | 1110.1193v2 |
2011-10-31 | Calculation of coercivity of magnetic nanostructures at finite temperatures | We report a finite temperature micromagnetic method (FTM) that allows for the
calculation of the coercive field of arbitrary shaped magnetic nanostructures
at time scales of nanoseconds to years. Instead of directly solving the
Landau-Lifshitz-Gilbert equation, the coercive field is obtained without any
free parameter by solving a non linear equation, which arises from the
transition state theory. The method is applicable to magnetic structures where
coercivity is determined by one thermally activated reversal or nucleation
process. The method shows excellent agreement with experimentally obtained
coercive fields of magnetic nanostructures and provides a deeper understanding
of the mechanism of coercivity. | 1110.6789v1 |
2011-11-24 | Spin-Wave Modes and Their Intense Excitation Effects in Skyrmion Crystals | We theoretically study spin-wave modes and their intense excitations
activated by microwave magnetic fields in the skyrmion-crystal phase of
insulating magnets by numerically analyzing a two-dimensional spin model using
the Landau-Lifshitz-Gilbert equation. Two peaks of spin-wave resonances with
frequencies of ~1 GHz are found for in-plane a.c. magnetic field where
distribution of the out-of-plane spin components circulates around each
skyrmion core. Directions of the circulations are opposite between these two
modes, and hence the spectra exhibit salient dependence on the circular
polarization of irradiating microwave. A breathing-type mode is also found for
out-of-plane a.c. magnetic field. By intensively exciting these collective
modes, melting of the skyrmion crystal accompanied by a red shift of the
resonant frequency is achieved within nano seconds. | 1111.5667v1 |
2011-11-26 | Energy dissipation and switching delay in stress-induced switching of multiferroic devices in the presence of thermal fluctuations | Switching the magnetization of a shape-anisotropic 2-phase multiferroic
nanomagnet with voltage-generated stress is known to dissipate very little
energy ($<$ 1 aJ for a switching time of $\sim$0.5 ns) at 0 K temperature.
Here, we show by solving the stochastic Landau-Lifshitz-Gilbert equation that
switching can be carried out with $\sim$100% probability in less than 1 ns
while dissipating less than 2 aJ at {\it room temperature}. This makes
nanomagnetic logic and memory systems, predicated on stress-induced magnetic
reversal, one of the most energy-efficient computing hardware extant. We also
study the dependence of energy dissipation, switching delay, and the critical
stress needed to switch, on the rate at which stress is ramped up or down. | 1111.6129v1 |
2011-11-30 | Multi-Weyl Topological Semimetals Stabilized by Point Group Symmetry | We perform a complete classification of two-band $\bk\cdot\mathbf{p}$
theories at band crossing points in 3D semimetals with $n$-fold rotation
symmetry and broken time-reversal symmetry. Using this classification, we show
the existence of new 3D topological semimetals characterized by
$C_{4,6}$-protected double-Weyl nodes with quadratic in-plane (along $k_{x,y}$)
dispersion or $C_6$-protected triple-Weyl nodes with cubic in-plane dispersion.
We apply this theory to the 3D ferromagnet HgCr$_2$Se$_4$ and confirm it is a
double-Weyl metal protected by $C_4$ symmetry. Furthermore, if the direction of
the ferromagnetism is shifted away from the [001]- to the [111]-axis, the
double-Weyl node splits into four single Weyl nodes, as dictated by the point
group $S_6$ of that phase. Finally, we discuss experimentally relevant effects
including splitting of multi-Weyl nodes by applying $C_n$ breaking strain and
the surface Fermi arcs in these new semimetals. | 1111.7309v2 |
2011-12-01 | Gate controlled Spin-Density Wave and Chiral FFLO Superconducting phases in interacting Quantum Spin Hall edge states | We explore the phases exhibited by an interacting quantum spin Hall edge
state in the presence of finite chemical potential (applied gate voltage) and
spin imbalance (applied magnetic field). We find that the helical nature of the
edge state gives rise to orders that are expected to be absent in non-chiral
one-dimensional electronic systems. For repulsive interactions, the ordered
state has an oscillatory spin texture whose ordering wavevector is controlled
by the chemical potential. We analyze the manner in which a magnetic impurity
provides signatures of such oscillations. We find that finite spin imbalance
favors a finite current carrying groundstate that is not condensed in the
absence of interactions and is superconducting for attractive interactions.
This state is characterized by FFLO-type oscillations where the Cooper pairs
obtain a finite center of mass momentum. | 1112.0289v1 |
2011-12-04 | A New Approach to Incremental Cycle Detection and Related Problems | We consider the problem of detecting a cycle in a directed graph that grows
by arc insertions, and the related problems of maintaining a topological order
and the strong components of such a graph. For these problems, we give two
algorithms, one suited to sparse graphs, and the other to dense graphs. The
former takes the minimum of O(m^{3/2}) and O(mn^{2/3}) time to insert m arcs
into an n-vertex graph; the latter takes O(n^2 log(n)) time. Our sparse
algorithm is considerably simpler than a previous O(m^{3/2})-time algorithm; it
is also faster on graphs of sufficient density. The time bound of our dense
algorithm beats the previously best time bound of O(n^{5/2}) for dense graphs.
Our algorithms rely for their efficiency on topologically ordered vertex
numberings; bounds on the size of the numbers give bound on running times. | 1112.0784v1 |
2011-12-08 | A 2-20 GHz Analog Lag-Correlator for Radio Interferometry | We present the design and testing of a 2-20 GHz continuum band analog lag
correlator with 16 frequency channels for astronomical interferometry. The
correlator has been designed for future use with a prototype single-baseline
interferometer operating at 185-275 GHz. The design uses a broadband Wilkinson
divider tree with integral thin-film resistors implemented on an alumina
substrate, and custom-made broadband InGaP/GaAs Gilbert Cell multipliers. The
prototype correlator has been fully bench-tested, together with the necessary
readout electronics for acquisition of the output signals. The results of these
measurements show that the response of the correlator is well behaved over the
band. An investigation of the noise behaviour also shows that the
signal-to-noise of the system is not limited by the correlator performance. | 1112.1982v1 |
2011-12-12 | An analysis of constraints on relativistic species from primordial nucleosynthesis and the cosmic microwave background | We present constraints on the number of relativistic species from a joint
analysis of cosmic microwave background (CMB) fluctuations and light element
abundances (helium and deuterium) compared to big bang nucleosynthesis (BBN)
predictions. Our BBN calculations include updates of nuclear rates in light of
recent experimental and theoretical information, with the most significant
change occuring for the d(p,gamma)^3He cross section. We calculate a likelihood
function for BBN theory and observations that accounts for both observational
errors and nuclear rate uncertainties and can be easily embedded in
cosmological parameter fitting. We then demonstrate that CMB and BBN are in
good agreement, suggesting that the number of relativistic species did not
change between the time of BBN and the time of recombination. The level of
agreement between BBN and CMB, as well as the agreement with the standard model
of particle physics, depends somewhat on systematic differences among
determinations of the primordial helium abundance. We demonstrate that
interesting constraints can be derived combining only CMB and D/H observations
with BBN theory, suggesting that an improved D/H constraint would be an
extremely valuable probe of cosmology. | 1112.2683v1 |
2011-12-22 | Higgs boson decay into 2 photons in the type~II Seesaw Model | We study the two photon decay channel of the Standard Model-like component of
the CP-even Higgs bosons present in the type II Seesaw Model. The corresponding
cross-section is found to be significantly enhanced in parts of the parameter
space, due to the (doubly-)charged Higgs bosons' $(H^{\pm \pm})H^\pm$ virtual
contributions, while all the other Higgs decay channels remain Standard
Model(SM)-like. In other parts of the parameter space $H^{\pm \pm}$ (and
$H^{\pm}$) interfere destructively, reducing the two photon branching ratio
tremendously below the SM prediction. Such properties allow to account for any
excess such as the one reported by ATLAS/CMS at $\approx 125$ GeV if confirmed
by future data; if not, for the fact that a SM-like Higgs exclusion in the
diphoton channel around 114-115 GeV as reported by ATLAS, does not contradict a
SM-like Higgs at LEP(!), and at any rate, for the fact that ATLAS/CMS exclusion
limits put stringent lower bounds on the $H^{\pm \pm}$ mass, particularly in
the parameter space regions where the direct limits from same-sign leptonic
decays of $H^{\pm \pm}$ do not apply. | 1112.5453v1 |
2012-01-18 | Cavity approach to sphere packing in Hamming space | In this paper we study the hard sphere packing problem in the Hamming space
by the cavity method. We show that both the replica symmetric and the replica
symmetry breaking approximations give maximum rates of packing that are
asymptotically the same as the lower bound of Gilbert and Varshamov.
Consistently with known numerical results, the replica symmetric equations also
suggest a crystalline solution, where for even diameters the spheres are more
likely to be found in one of the subspaces (even or odd) of the Hamming space.
These crystalline packings can be generated by a recursive algorithm which
finds maximum packings in an ultra-metric space. Finally, we design a message
passing algorithm based on the cavity equations to find dense packings of hard
spheres. Known maximum packings are reproduced efficiently in non trivial
ranges of dimensions and number of spheres. | 1201.3863v2 |
2012-01-19 | Impact of microstructure, temperature and strain ratio on energy-based low- cycle fatigue life prediction models for TiAl alloys | In this paper, two fatigue lifetime prediction models are tested on TiAl
intermetallic using results from uniaxial low-cycle fatigue tests. Both
assessments are based on dissipated energy but one of them considers a
hydrostatic pressure correction. This work allows to confirm, on this kind of
material, the linear nature, already noticed on silicon molybdenum cast iron,
TiNi shape memory alloy and 304L stainless steel, of dissipated energy,
corrected or not with hydrostatic pressure, according to the number of cycles
to failure. This study also highlights that, firstly, the dissipated energy
model is here more adequate to estimate low-cycle fatigue life and that,
secondly, intrinsic parameters like microstructure as well as extrinsic
parameters like temperature or strain ratio have an impact on prediction
results. | 1201.4084v1 |
2012-01-23 | Influence of the magnetoelectric coupling on the electric field induced magnetization reversal in a composite non-strained multiferroic chain | We study theoretically the multiferroic dynamics in a composite
one-dimensional system consisting of BaTiO3 multiferroically coupled to an iron
chain. The method treats the magnetization and the polarization as
thermodynamic quantities describable via a combination of the Landau-Lifshits-
Gilbert and the Ginzburg-Landau dynamics coupled via an additional term in the
total free energy density. This term stems from the multiferroic interaction at
the interface. For a wide range of strengths of this coupling we predict the
possibility of obtaining a well-developed hysteresis in the ferromagnetic part
of the system induced by an external electric field. The dependence of the
reversal modes on the electric field frequency is also investigated and we
predict a considerable stability of the magnetization reversal for frequencies
in the range of 0.5 - 12 [GHz]. | 1201.4740v1 |
2012-01-27 | Domain-wall complexes in ferromagnetic stripes | Interaction of domain walls (DWs) in ferromagnetic stripes is studied with
relevance to the formation of stable complexes of many domains. Two DW system
is described with the Landau-Lifshitz-Gilbert equation including regimes of
narrow and wide stripes which correspond the presence of transverse and vortex
DWs. The DWs of both kinds are characterized with their chiralities (the
direction of the magnetization rotation in the stripe plane) and polarities
(the magnetization orientation in the center of a vortex and/or halfvortices),
hence, their interactions are analyzed with dependence on these properties. In
particular, pairs of the DWs of opposite or like both chiralities and
polarities are investigated as well as pairs of opposite (like) chiralities and
of like (opposite) polarities. Conditions of the creation of stationary
magnetic bubbles built of two interacting DWs are formulated with relevance to
the situations of presence and absence of the external magnetic field. | 1201.5760v3 |
2012-01-31 | Interlayer Transport in Disordered Semiconductor Electron Bilayers | We study the effects of disorder on the interlayer transport properties of
disordered semiconductor bilayers outside of the quantum Hall regime by
performing self-consistent quantum transport calculations. We find that the
addition of material disorder to the system affects interlayer interactions
leading to significant deviations in the interlayer transfer characteristics.
In particular, we find that disorder decreases and broadens the tunneling peak,
effectively reducing the interacting system to the non-interacting system, when
the mean-free path for the electrons becomes shorter than the system length.
Our results suggest that the experimental observation of exchange-enhanced
interlayer transport in semiconductor bilayers requires materials with
mean-free paths larger than the spatial extent of the system. | 1202.0053v1 |
2012-02-01 | Imaging topologically protected transport with quantum degenerate gases | Ultracold and quantum degenerate gases held near conductive surfaces can
serve as sensitive, high resolution, and wide-area probes of electronic current
flow. Previous work has imaged transport around grain boundaries in a gold wire
by using ultracold and Bose-Einstein condensed atoms held microns from the
surface with an atom chip trap. We show that atom chip microscopy may be
applied to useful purpose in the context of materials exhibiting topologically
protected surface transport. Current flow through lithographically tailored
surface defects in topological insulators (TI)---both idealized and with the
band-structure and conductivity typical of Bi$_{2}$Se$_{3}$---is numerically
calculated. We propose that imaging current flow patterns enables the
differentiation of an ideal TI from one with a finite bulk--to--surface
conductivity ratio, and specifically, that the determination of this ratio may
be possible by imaging transport around trenches etched into the TI's surface. | 1202.0060v2 |
2012-02-02 | Large deviations and transitions between equilibria for stochastic Landau-Lifshitz-Gilbert equation | We study a stochastic Landau-Lifshitz equation on a bounded interval and with
finite dimensional noise. We first show that there exists a pathwise unique
solution to this equation and that this solution enjoys the maximal regularity
property. Next, we prove the large deviations principle for small noise
asymptotic of solutions using the weak convergence method. An essential
ingredient of the proof is compactness, or weak to strong continuity, of the
solution map for a deterministic Landau-Lifschitz equation, when considered as
a transformation of external fields. We then apply this large deviations
principle to show that small noise can cause magnetisation reversal. We also
show the importance of the shape anisotropy parameter for reducing the
disturbance of the solution caused by small noise. The problem is motivated by
applications of ferromagnetic nanowires to the fabrication of magnetic
memories.
This is an updated version of the previous version of this paper. | 1202.0370v2 |
2012-02-06 | Atomic level micromagnetic model of recording media switching at elevated temperatures | An atomic level micromagnetic model of granular recording media is developed
and applied to examine external field-induced grain switching at elevated
temperatures which captures non-uniform reversal modes. The results are
compared with traditional methods which employ the Landau-Lifshitz-Gilbert
equations based on uniformly magnetized grains with assigned intrinsic
temperature profiles for $M(T)$ and $K(T)$. Using nominal parameters
corresponding to high-anisotropy FePt-type media envisioned for Energy Assisted
Magnetic Recording, our results demonstrate that atomic-level reversal slightly
reduces the field required to switch grains at elevated temperatures, but
results in larger fluctuations, when compared to a uniformly magnetized grain
model. | 1202.1131v1 |
2012-02-10 | A model of magnetic order in hexagonal HoMnO3 | Symmetry arguments are used to develop a spin Hamiltonian for the description
of the complex magnetic ordering in HoMnO$_3$. Using a novel application of the
Landau Lifshitz Gilbert dynamic torque equations to this model of the
frustrated Mn ions on an $AB$ stacked triangular antiferromagnetic, it is shown
that the four principal spin configurations observed in this compound are
stabilized. Ho-Mn coupling is found to be a consequence of an unusual trigonal
anisotropy term which is responsible for simultaneous Mn spin reorientation and
onset of Ho magnetic order. Based on these microscopic considerations, a
mean-field Landau-type free energy is derived which reproduces the succession
of observed temperature driven magnetic phase transitions at zero field,
including re-entrant behavior. In addition, our analysis suggests that the
basal-plane magnetic order should be slightly incommensurate with the lattice. | 1202.2321v1 |
2012-02-13 | Switching Distributions for Perpendicular Spin-Torque Devices within the Macrospin Approximation | We model "soft" error rates for writing (WSER) and for reading (RSER) for
perpendicular spin-torque memory devices by solving the Fokker-Planck equation
for the probability distribution of the angle that the free layer magnetization
makes with the normal to the plane of the film. We obtain: (1) an exact, closed
form, analytical expression for the zero-temperature switching time as a
function of initial angle; (2) an approximate analytical expression for the
exponential decay of the WSER as a function of the time the current is applied;
(3) comparison of the approximate analytical expression for the WSER to
numerical solutions of the Fokker-Planck equation; (4) an approximate
analytical expression for the linear increase in RSER with current applied for
reading; (5) comparison of the approximate analytical formula for the RSER to
the numerical solution of the Fokker-Planck equation; and (6) confirmation of
the accuracy of the Fokker-Planck solutions by comparison with results of
direct simulation using the single-macrospin Landau-Lifshitz-Gilbert (LLG)
equations with a random fluctuating field in the short-time regime for which
the latter is practical. | 1202.2621v1 |
2012-02-13 | Error- and Loss-Tolerances of Surface Codes with General Lattice Structures | We propose a family of surface codes with general lattice structures, where
the error-tolerances against bit and phase errors can be controlled
asymmetrically by changing the underlying lattice geometries. The surface codes
on various lattices are found to be efficient in the sense that their threshold
values universally approach the quantum Gilbert-Varshamov bound. We find that
the error-tolerance of surface codes depends on the connectivity of underlying
lattices; the error chains on a lattice of lower connectivity are easier to
correct. On the other hand, the loss-tolerance of surface codes exhibits an
opposite behavior; the logical information on a lattice of higher connectivity
has more robustness against qubit loss. As a result, we come upon a fundamental
trade-off between error- and loss-tolerances in the family of the surface codes
with different lattice geometries. | 1202.2743v1 |
2012-02-28 | A method for exploratory repeated-measures analysis applied to a breast-cancer screening study | When a model may be fitted separately to each individual statistical unit,
inspection of the point estimates may help the statistician to understand
between-individual variability and to identify possible relationships. However,
some information will be lost in such an approach because estimation
uncertainty is disregarded. We present a comparative method for exploratory
repeated-measures analysis to complement the point estimates that was motivated
by and is demonstrated by analysis of data from the CADET II breast-cancer
screening study. The approach helped to flag up some unusual reader behavior,
to assess differences in performance, and to identify potential random-effects
models for further analysis. | 1202.6133v1 |
2012-03-15 | Size bias and differential lensing of strongly lensed, dusty galaxies identified in wide-field surveys | We address two selection effects that operate on samples of gravitationally
lensed dusty galaxies identified in millimeter- and submillimeter-wavelength
surveys. First, we point out the existence of a "size bias" in such samples:
due to finite source effects, sources with higher observed fluxes are
increasingly biased towards more compact objects. Second, we examine the effect
of differential lensing in individual lens systems by modeling each source as a
compact core embedded in an extended diffuse halo. Considering the ratio of
magnifications in these two components, we find that at high overall
magnifications the compact component is amplified by a much larger factor than
the diffuse component, but at intermediate magnifications (~10) the probability
of a larger magnification for the extended region is higher. Lens models
determined from multi-frequency resolved imaging data are crucial to correct
for this effect. | 1203.3267v2 |
2012-03-28 | Decoherence and mode-hopping in a magnetic tunnel junction-based spin-torque oscillator | We discuss the coherence of magnetic oscillations in a magnetic tunnel
junction-based spin-torque oscillator as a function of external field angle.
Time-frequency analysis shows mode-hopping between distinct oscillator modes,
which arises from linear and nonlinear couplings in the Landau-Lifshitz-Gilbert
equation, analogous to mode-hopping observed in semiconductor ring lasers.
These couplings and therefore mode-hopping are minimized near the current
threshold for antiparallel (AP) alignment of free layer with reference layer
magnetization. Away from the AP alignment, mode-hopping limits oscillator
coherence. | 1203.6300v1 |
2012-04-28 | Topological Excitonic Superfluids in Three Dimensions | We study the equilibrium and non-equilibrium properties of topological
dipolar intersurface exciton condensates within time-reversal invariant
topological insulators in three spatial dimensions without a magnetic field. We
elucidate that, in order to correctly identify the proper pairing symmetry
within the condensate order parameter, the full three-dimensional Hamiltonian
must be considered. As a corollary, we demonstrate that only particles with
similar chirality play a significant role in condensate formation. Furthermore,
we find that the intersurface exciton condensation is not suppressed by the
interconnection of surfaces in three-dimensional topological insulators as the
intersurface polarizability vanishes in the condensed phase. This eliminates
the surface current flow leaving only intersurface current flow through the
bulk. We conclude by illustrating how the excitonic superfluidity may be
identified through an examination of the terminal currents above and below the
condensate critical current. | 1204.6351v2 |
2012-05-29 | Thermally-Assisted Spin-Transfer Torque Magnetization Reversal in Uniaxial Nanomagnets | We simulate the stochastic Landau-Lifshitz-Gilbert (LLG) dynamics of a
uniaxial nanomagnet out to sub-millisecond timescales using a graphical
processing unit based micromagnetic code and determine the effect of
geometrical tilts between the spin-current and uniaxial anisotropy axes on the
thermally assisted reversal dynamics. The asymptotic behavior of the switching
time ($I\rightarrow 0$, $<\tau>\propto\exp(-\xi(1-I)^2)$) is approached
gradually, indicating a broad crossover regime between ballistic and thermally
assisted spin transfer reversal. Interestingly, the mean switching time is
shown to be nearly independent of the angle between the spin current and
magnet's uniaxial axes. These results have important implications for modeling
the energetics of thermally assisted magnetization reversal of spin transfer
magnetic random access memory bit cells. | 1205.6509v1 |
2012-06-06 | Fractional Spin Josephson Effect and Electrically Controlled Magnetization in Quantum Spin Hall Edges | We explore a spin Josephson effect in a system of two ferromagnets coupled by
a tunnel junction formed of 2D time-reversal invariant topological insulators.
In analogy with the more commonly studied instance of the Josephson effect for
charge in superconductors, we investigate properties of the phase-coherent {\it
spin} current resulting from the misalignment of the in-plane magnetization
angles of the two ferromagnets. We show that the topological insulating barrier
offers the exciting prospect of hosting a {\it fractional} spin Josephson
effect mediated by bound states at the ferromagnet-topological insulator
interface. We provide multiple perspectives to understand the $4\pi$ periodic
nature of this effect. We discuss several measurable consequences, such as, the
generation of a transverse voltage signal which allows for purely electrical
measurements, an inverse of this effect where an applied voltage gives rise to
a transverse spin-current, and a fractional AC spin-Josephson effect. | 1206.1295v2 |
2012-06-07 | Magnetic particle hyperthermia: Power losses under circularly polarized field in anisotropic nanoparticles | The deterministic Landau-Lifshitz-Gilbert equation has been used to
investigate the nonlinear dynamics of magnetization and the specific loss power
in magnetic nanoparticles with uniaxial anisotropy driven by a rotating
magnetic field, generalizing the results obtained for the isotropic case found
in [P. F. de Chatel, I. Nandori, J. Hakl, S. Meszaros and K. Vad, J. Phys.:
Condens. Matter 21, 124202 (2009)]. As opposed to many applications of
magnetization reversal in single-domain ferromagnetic particles where losses
must be minimized, in this paper, we study the mechanisms of dissipation used
in cancer therapy by hyperthermia which requires the enhancement of energy
losses. We show that for circularly polarized field, the loss energy per cycle
is decreased by the anisotropy compared to the isotropic case when only
dynamical effects are taken into account. Thus, in this case, in the low
frequency limit, a better heating efficiency can be achieved for isotropic
nanoparticles. The possible role of thermal fluctuations is also discussed.
Results obtained are compared to experimental data. | 1206.1544v2 |
2012-06-27 | Searching for Fast Optical Transients using VERITAS Cherenkov Telescopes | Astronomical transients are intrinsically interesting things to study. Fast
optical transients (microsecond timescale) are a largely unexplored field of
optical astronomy mainly due to the fact that large optical telescopes are
oversubscribed. Furthermore, most optical observations use instruments with
integration times on the order of seconds and are thus unable to resolve fast
transients. Current-generation atmospheric Cherenkov gamma-ray telescopes,
however, have huge collecting areas (e.g., VERITAS, which consists of four 12-m
telescopes), larger than any existing optical telescopes, and time is typically
available for such studies without interfering with gamma-ray observations. The
following outlines the benefits of using a Cherenkov telescope to detect
optical transients and the implementation of the VERITAS Transient Detector
(TRenDy), a dedicated multi-channel photometer based on field-programmable gate
arrays. Data are presented demonstrating the ability of TRenDy to detect
transient events such as a star passing through its field of view and the
optical light curve of a pulsar. | 1206.6535v1 |
2012-07-05 | Perpendicular Magnetization and Generic Realization of the Ising Model in Artificial Spin Ice | We have studied frustrated kagome arrays and unfrustrated honeycomb arrays of
magnetostatically-interacting single-domain ferromagnetic islands with
magnetization normal to the plane. The measured pairwise spin correlations of
both lattices can be reproduced by models based solely on nearest-neighbor
correlations. The kagome array has qualitatively different magnetostatics but
identical lattice topology to previously-studied 'artificial spin ice' systems
composed of in-plane moments. The two systems show striking similarities in the
development of moment pair correlations, demonstrating a universality in
artificial spin ice behavior independent of specific realization in a
particular material system. | 1207.1456v1 |
2012-07-13 | Magnetic vortices induced by a moving tip | A two-dimensional easy-plane ferromagnetic substrate, interacting with a
dipolar tip which is magnetised perpendicular with respect to the easy plane is
studied numerically by solving the Landau-Lifshitz Gilbert equation. Due to the
symmetry of the dipolar field of the tip, in addition to the collinear
structure a magnetic vortex structure becomes stable. It is robust against
excitations caused by the motion of the tip. We show that for high excitations
the system may perform a transition between the two states. The influence of
domain walls, which may also induce this transition, is examined. | 1207.3225v2 |
2012-07-24 | Bulk Topological Invariants in Noninteracting Point Group Symmetric Insulators | We survey various quantized bulk physical observables in two- and
three-dimensional topological band insulators invariant under translational
symmetry and crystallographic point group symmetries (PGS). In two-dimensional
insulators, we show that: (i) the Chern number of a $C_n$-invariant insulator
can be determined, up to a multiple of $n$, by evaluating the eigenvalues of
symmetry operators at high-symmetry points in the Brillouin zone; (ii) the
Chern number of a $C_n$-invariant insulator is also determined, up to a
multiple of $n$, by the $C_n$ eigenvalue of the Slater determinant of a
noninteracting many-body system and (iii) the Chern number vanishes in
insulators with dihedral point groups $D_n$, and the quantized electric
polarization is a topological invariant for these insulators. In
three-dimensional insulators, we show that: (i) only insulators with point
groups $C_n$, $C_{nh}$ and $S_n$ PGS can have nonzero 3D quantum Hall
coefficient and (ii) only insulators with improper rotation symmetries can have
quantized magnetoelectric polarization $P_3$ in the term
$P_3\mathbf{E}\cdot\mathbf{B}$, the axion term in the electrodynamics of the
insulator (medium). | 1207.5767v2 |
2012-07-30 | 'MOHAWK' : a 4000-fiber positioner for DESpec | We present a concept for a 4000-fibre positioner for DESpec, based on the
Echidna 'tilting spine' technology. The DESpec focal plane is 450mm across and
curved, and the required pitch is ~6.75mm. The size, number of fibers and
curvature are all comparable with various concept studies for similar
instruments already undertaken at the AAO, but present new challenges in
combination. A simple, low-cost, and highly modular design is presented,
consisting of identical modules populated by identical spines. No show-stopping
issues in accommodating either the curvature or the smaller pitch have been
identified, and the actuators consist largely of off-the-shelf components. The
actuators have been prototyped at AAO, and allow reconfiguration times of ~15s
to reach position errors 7 microns or less. Straightforward designs for
metrology, acquisition, and guiding are also proposed. The throughput losses of
the entire positioner system are estimated to be ~15%, of which 6.3% is
attributable to the tilting-spine technology. | 1207.7011v2 |
2012-07-31 | Throughput of Rateless Codes over Broadcast Erasure Channels | In this paper, we characterize the throughput of a broadcast network with n
receivers using rateless codes with block size K. We assume that the underlying
channel is a Markov modulated erasure channel that is i.i.d. across users, but
can be correlated in time. We characterize the system throughput asymptotically
in n. Specifically, we explicitly show how the throughput behaves for different
values of the coding block size K as a function of n, as n approaches infinity.
For finite values of K and n, under the more restrictive assumption of
Gilbert-Elliott channels, we are able to provide a lower bound on the maximum
achievable throughput. Using simulations we show the tightness of the bound
with respect to system parameters n and K, and find that its performance is
significantly better than the previously known lower bounds. | 1207.7298v1 |
2012-08-01 | Streaming Codes for Channels with Burst and Isolated Erasures | We study low-delay error correction codes for streaming recovery over a class
of packet-erasure channels that introduce both burst-erasures and isolated
erasures. We propose a simple, yet effective class of codes whose parameters
can be tuned to obtain a tradeoff between the capability to correct burst and
isolated erasures. Our construction generalizes previously proposed low-delay
codes which are effective only against burst erasures. We establish an
information theoretic upper bound on the capability of any code to
simultaneously correct burst and isolated erasures and show that our proposed
constructions meet the upper bound in some special cases. We discuss the
operational significance of column-distance and column-span metrics and
establish that the rate 1/2 codes discovered by Martinian and Sundberg [IT
Trans.\, 2004] through a computer search indeed attain the optimal
column-distance and column-span tradeoff. Numerical simulations over a
Gilbert-Elliott channel model and a Fritchman model show significant
performance gains over previously proposed low-delay codes and random linear
codes for certain range of channel parameters. | 1208.0072v1 |
2012-08-23 | Inconsistencies in the description of pairing effects in nuclear level densities | Pairing correlations have a strong influence on nuclear level densities.
Empirical descriptions and theoretical models have been developed to take these
effects into account. The present article discusses cases, where descriptions
of nuclear level densities are inconsistent or in conflict with the present
understanding of nuclear properties. Phenomenological approaches consider a
back-shift parameter. However, the absolute magnitude of the back-shift, which
actually corresponds to the pairing condensation energy, is generally not
compatible with the observation that stable pairing correlations are present in
essentially all nuclei. It is also shown that in the BCS model pairing
condensation energies and critical pairing energies are inconsistent for light
nuclei. A modification to the composite Gilbert-Cameron level-density
description is proposed, and the use of more realistic pairing theories is
suggested. | 1208.4709v1 |
2012-09-26 | Symmetry Protected Josephson Supercurrents in Three-Dimensional Topological Insulators | Coupling the surface state of a topological insulator (TI) to an s-wave
superconductor is predicted to produce the long-sought Majorana quasiparticle
excitations. However, superconductivity has not been measured in surface states
when the bulk charge carriers are fully depleted, i.e., in the true topological
regime that is relevant for investigating Majorana modes. Here, we report
measurements of DC Josephson effects in TI-superconductor junctions as the
chemical potential is moved from the bulk bands into the band gap, or through
the true topological regime characterized by the presence of only surface
currents. We examine the relative behavior of the system at different
bulk/surface ratios, determining the effects of strong bulk/surface mixing,
disorder, and magnetic field. We compare our results to 3D quantum transport
simulations to conclude that the supercurrent is largely carried by surface
states, due to the inherent topology of the bands, and that it is robust
against disorder. | 1209.5830v2 |
2012-10-09 | A microscopic model for ultrafast remagnetization dynamics | In this letter, we provide a microscopic model for the ultrafast
remagnetization of atomic moments already quenched above Stoner-Curie
temperature by a strong laser fluence. Combining first principles density
functional theory, atomistic spin dynamics utilizing the
Landau-Lifshitz-Gilbert equation and a three temperature model, we show the
temporal evolution of atomic moments as well as the macroscopic magnetization
of bcc Fe and hcp Co covering a broad time scale, ranging from femtoseconds to
picoseconds. Our simulations show a variety of complex temporal behavior of the
magnetic properties resulting from an interplay between electron, spin and
lattice subsystems, which causes an intricate time evolution of the atomic
moment, where longitudinal and transversal fluctuations result in a macro spin
moment that evolves non-monotonically. | 1210.2616v1 |
2012-10-31 | Theory and Simulation of the diffusion of kinks on dislocations in bcc metals | Isolated kinks on thermally fluctuating (1/2)<111> screw, <100> edge and
(1/2)<111> edge dislocations in bcc iron are simulated under zero stress
conditions using molecular dynamics (MD). Kinks are seen to perform stochastic
motion in a potential landscape that depends on the dislocation character and
geometry, and their motion provides fresh insight into the coupling of
dislocations to a heat bath. The kink formation energy, migration barrier and
friction parameter are deduced from the simulations. A discrete
Frenkel-Kontorova-Langevin (FKL) model is able to reproduce the coarse grained
data from MD at a fraction of the computational cost, without assuming an a
priori temperature dependence beyond the fluctuation-dissipation theorem.
Analytic results reveal that discreteness effects play an essential r\^ole in
thermally activated dislocation glide, revealing the existence of a crucial
intermediate length scale between molecular and dislocation dynamics. The model
is used to investigate dislocation motion under the vanishingly small stress
levels found in the evolution of dislocation microstructures in irradiated
materials. | 1210.8327v2 |
2012-11-01 | Dynamically-generated pure spin current in single-layer graphene | The conductance mismatch problem limits the spin-injection efficiency
significantly, and spin-injection into graphene has been usually requiring
high-quality tunnel barriers to circumvent the conductance mismatch. We
introduce a novel approach, which enables generation of a pure spin current
into single-layer graphene (SLG) free from electrical conductance mismatch by
using dynamical spin injection. Experimental demonstration of
spin-pumping-induced spin current generation and spin transport in SLG at room
temperature was successfully achieved and the spin coherence was estimated to
be 1.36 {\mu}m by using a conventional theoretical model based on
Landau-Lifshitz-Gilbert equation. The spin coherence is proportional to the
quality of SLG, which indicates that spin relaxation in SLG is governed by the
Elliot-Yafet mechanism as was reported. | 1211.0124v1 |
2012-12-04 | Phenomenological theory of the giant magnetoimpedance of composite wires | Composite wires with a three-layered structure are known to show a
particularly large magnetoimpedance effect. The wires consist of a highly
conductive core, an insulating layer and an outer ferromagnetic shell. In order
to understand the origin of the effect a theory based on a coupling of the
Maxwell equations to the Landau-Lifschitz-Gilbert equation is suggested. The
theory is phenomenological in the sense that it does not account for a domain
structure. However, theoretical results nicely reproduce those obtained in
various measurements. Furthermore, an upper limit of the magnetoimpedance ratio
for a given combination of materials can be determined. | 1212.0710v3 |
2012-12-12 | Giant magnetoimpedance of composite wires with an insulation layer | Composite wires with a three-layered structure exhibit a large giant
magneto-impedance (GMI) effect, which can be used in sensitive magnetic field
sensors. To further investigate the origin of the GMI effect, composite wires
consisting of a highly conductive copper core, a silicon dioxide layer and an
outer Permalloy shell were prepared by radio frequency (RF) magnetron
sputtering. The GMI ratio was measured at various driving current frequencies
and with different insulating layer thicknesses. A theoretical model by
coupling the Maxwell equations to the Landau-Lifschitz-Gilbert equation was
developed to investigate the composite wire impedance and its dependence on
external magnetic field, current frequency and insulating layer thickness.
Experimental results corroborate the theoretical model. | 1212.2745v2 |
2012-12-13 | Theory of quasiparticle interference in mirror symmetric 2D systems and its application to surface states of topological crystalline insulators | We study symmetry protected features in the quasiparticle interference (QPI)
pattern of 2D systems with mirror symmetries and time-reversal symmetry, around
a single static point impurity. We show that, in the Fourier transformed local
density of states (FT-LDOS), $\rho(\bq,\omega)$, while the position of high
intensity peaks generically depends on the geometric features of the iso-energy
contour at energy $\omega$, the \emph{absence} of certain peaks is guaranteed
by the opposite mirror eigenvalues of the two Bloch states that are (i) on the
mirror symmetric lines in the Brillouin zone (BZ) and (ii) separated by
scattering vector $\bq$. We apply the general result to the QPI on the $ <{001}
>$-surface of topological crystalline insulator Pb$_{1-x}$Sn$_x$Te and predict
all vanishing peaks in $\rho(\bq,\omega)$. The model-independent analysis is
supported by numerical calculations using an effective four-band model derived
from symmetry analysis. | 1212.3285v2 |
2013-01-25 | Chiral magnetism and helimagnons in a pyrochlore antiferromagnet | Recent neutron scattering measurements on the spinel CdCr2O4 revealed a rare
example of helical magnetic order in geometrically frustrated pyrochlore
antiferromagnet. The spin spiral characterized by an incommensurate wavevector
Q = 2pi (0, delta, 1) with delta ~0.09 is accompanied by a tetragonal
distortion. Here we conduct a systematic study on the magnetic ground state
resulting from the interplay between the Dzyaloshinskii-Moriya interaction and
further neighbor exchange couplings, two of the most important mechanisms for
stabilizing incommensurate spin orders. We compute the low-energy spin-wave
spectrum based on a microscopic spin Hamiltonian and find a dispersion relation
characteristic of the helimagnons. By numerically integrating the
Landau-Lifshitz-Gilbert equation with realistic model parameters, an overall
agreement between experiment and the numerical spectrum, lending further
support to the view that a softened optical phonon triggers the magnetic
transition and endows the lattice a chirality. | 1301.5958v1 |
2013-01-31 | Switching of Dipole Coupled Multiferroic Nanomagnets in the Presence of Thermal Noise: Reliability of Nanomagnetic Logic | The stress-induced switching behavior of a multiferroic nanomagnet, dipole
coupled to a hard nanomagnet, is numerically studied by solving the stochastic
Landau-Lifshitz-Gilbert (LLG) equation for a single domain macro-spin state.
Different factors were found to affect the switching probability in the
presence of thermal noise at room temperature: (i) dipole coupling strength,
(ii) stress levels, and (iii) stress withdrawal rates (ramp rates). We report
that the thermal broadening of the magnetization distribution causes large
switching error rates. This could render nanomagnetic logic schemes that rely
on dipole coupling to perform Boolean logic operations impractical whether they
are clocked by stress or field or other means. | 1301.7490v2 |
2013-02-01 | Vortex mechanics in planar nano-magnets | A collective-variable approach for the study of non-linear dynamics of
magnetic textures in planar nano-magnets is proposed. The variables are just
arbitrary parameters (complex or real) in the specified analytical function of
a complex variable, describing the texture in motion. Starting with such a
function, a formal procedure is outlined, allowing a (non-linear) system of
differential equations of motion to be obtained for the variables. The
resulting equations are equivalent to Landau-Lifshitz-Gilbert dynamics as far
as the definition of collective variables allows it. Apart from the
collective-variable specification, the procedure does not involve any
additional assumptions (such as translational invariance or steady-state
motion). As an example, the equations of weakly non-linear motion of a magnetic
vortex are derived and solved analytically. A simple formula for the dependence
of the vortex precession frequency on its amplitude is derived. The results are
verified against special cases from the literature and agree quantitatively
with experiments and simulations. | 1302.0222v3 |
2013-02-03 | Correcting Camera Shake by Incremental Sparse Approximation | The problem of deblurring an image when the blur kernel is unknown remains
challenging after decades of work. Recently there has been rapid progress on
correcting irregular blur patterns caused by camera shake, but there is still
much room for improvement. We propose a new blind deconvolution method using
incremental sparse edge approximation to recover images blurred by camera
shake. We estimate the blur kernel first from only the strongest edges in the
image, then gradually refine this estimate by allowing for weaker and weaker
edges. Our method competes with the benchmark deblurring performance of the
state-of-the-art while being significantly faster and easier to generalize. | 1302.0439v2 |
2013-03-01 | Entropy Distance | Motivated by the approach of random linear codes, a new distance in the
vector space over a finite field is defined as the logarithm of the "surface
area" of a Hamming ball with radius being the corresponding Hamming distance.
It is named entropy distance because of its close relation with entropy
function. It is shown that entropy distance is a metric for a non-binary field
and a pseudometric for the binary field. The entropy distance of a linear code
is defined to be the smallest entropy distance between distinct codewords of
the code. Analogues of the Gilbert bound, the Hamming bound, and the Singleton
bound are derived for the largest size of a linear code given the length and
entropy distance of the code. Furthermore, as an important property related to
lossless joint source-channel coding, the entropy distance of a linear encoder
is defined. Very tight upper and lower bounds are obtained for the largest
entropy distance of a linear encoder with given dimensions of input and output
vector spaces. | 1303.0070v2 |
2013-03-20 | On the Behavior of RObust Header Compression U-mode in Channels with Memory | The existing studies of RObust Header Compression (ROHC) have provided some
understanding for memoryless channel, but the behavior of ROHC for correlated
wireless channels is not well investigated in spite of its practical
importance. In this paper, the dependence of ROHC against its design parameters
for the Gilbert Elliot channel is studied by means of three analytical models.
A first more elaborated approach accurately predicts the behavior of the
protocol for the single RTP flow profile, while a simpler, analytically
tractable model yields clear and insightful mathematical relationships that
explain the qualitative trends of ROHC. The results are validated against a
real world implementation of this protocol. Moreover, a third model studies
also the less conventional yet practically relevant setting of multiple RTP
flows. | 1303.4970v1 |
2013-03-21 | Tuning magnetic anisotropy in (001) oriented L10 (Fe1-xCux)55Pt45 films | We have achieved (001) oriented L10 (Fe1-xCux)55Pt45 thin films, with
magnetic anisotropy up to 3.6x10^7 erg/cm^3, using atomic-scale multilayer
sputtering and post annealing at 400 {\deg}C for 10 seconds. By fixing the Pt
concentration, structure and magnetic properties are systematically tuned by
the Cu addition. Increasing Cu content results in an increase in the tetragonal
distortion of the L10 phase, significant changes to the film microstructure,
and lowering of the saturation magnetization and anisotropy. The relatively
convenient synthesis conditions, along with the tunable magnetic properties,
make such materials highly desirable for future magnetic recording
technologies. | 1303.5208v1 |
2013-03-28 | Spin Torque on Magnetic Textures Coupled to the Surface of a Three-Dimensional Topological Insulator | We investigate theoretically the spin torque and magnetization dynamic in a
thin ferromagnetic (FM) layer with spatially varying magnetization. The FM
layer is deposited on the surface of a topological insulator (TI). In the limit
of the adiabatic relaxation of electron spin along the magnetization, the
interaction between the exchange interaction and the Rashba-like surface
texture of a TI yields a topological gauge field. Under the gauge field and an
applied current, spin torque is induced according to the direction of the
current. We derived the corresponding effective anisotropy field and hence the
modified Landau-Lifshitz-Gilbert equation, which describes the spin torque and
the magnetization dynamic. In addition, we study the effective field for
exemplary magnetic textures, such as domain wall, skyrmion, and vortex
configurations. The estimated strength of the effective field is comparable to
the switching fields of typical FM materials, and hence can significantly
influence the dynamics of the FM layer. | 1303.7031v1 |
2013-04-03 | Synchronization of an array of spin torque nano oscillators in periodic applied external magnetic field | Considering an array of spin torque transfer nano oscillators (STNOs), we
have investigated the synchronization property of the system under the action
of a common periodically driven applied external magnetic field by numerically
analyzing the underlying system of Landau-Lifshitz-Gilbert-Slonczewski (LLGS)
equations for the macro-spin variables. We find the novel result that the
applied external magnetic field can act as a medium to induce synchronization
of periodic oscillations, both in-phase and anti-phase, even without coupling
through spin current, thereby leading to the exciting possibility of
enhancement of microwave power in a straightforward way. | 1304.0875v1 |
2013-04-06 | Towards a living earth simulator | The Living Earth Simulator (LES) is one of the core components of the
FuturICT architecture. It will work as a federation of methods, tools,
techniques and facilities supporting all of the FuturICT simulation-related
activities to allow and encourage interactive exploration and understanding of
societal issues. Society-relevant problems will be targeted by leaning on
approaches based on complex systems theories and data science in tight
interaction with the other components of FuturICT. The LES will evaluate and
provide answers to real-world questions by taking into account multiple
scenarios. It will build on present approaches such as agent-based simulation
and modeling, multiscale modelling, statistical inference, and data mining,
moving beyond disciplinary borders to achieve a new perspective on complex
social systems. | 1304.1903v1 |
2013-05-13 | Backhopping effect in magnetic tunnel junctions: comparison between theory and experiment | We report on the magnetic switching and backhopping effects due to
spin-transfer-torque in magnetic tunnel junctions. Experimental data on the
current-induced switching in junctions with MgO tunnel barrier reveal a random
back-and-forth switching between the magnetization states, which appears when
the current direction favors the parallel magnetic configuration. The effect
depends on the barrier thickness $t_b$, and is not observed in tunnel junctions
with very thin MgO tunnel barriers, $t_b$ $<$ 0.95 nm. Switching dependence on
the bias voltage and barrier thickness is explained in terms of the macrospin
model, with the magnetization dynamics described by the modified
Landau-Lifshitz-Gilbert equation. Numerical simulations indicate that the
competition between in-plane and out-of-plane torque components can result at
high bias voltages in a non-deterministic switching behavior, in agreement with
experimental observations. When the barrier thickness is reduced, the overall
coupling between the magnetic layers across the barrier becomes ferromagnetic,
which suppresses the backhopping effect. | 1305.2711v1 |
2013-05-15 | Robust Streaming Erasure Codes based on Deterministic Channel Approximations | We study near optimal error correction codes for real-time communication. In
our setup the encoder must operate on an incoming source stream in a sequential
manner, and the decoder must reconstruct each source packet within a fixed
playback deadline of $T$ packets. The underlying channel is a packet erasure
channel that can introduce both burst and isolated losses.
We first consider a class of channels that in any window of length ${T+1}$
introduce either a single erasure burst of a given maximum length $B,$ or a
certain maximum number $N$ of isolated erasures. We demonstrate that for a
fixed rate and delay, there exists a tradeoff between the achievable values of
$B$ and $N,$ and propose a family of codes that is near optimal with respect to
this tradeoff. We also consider another class of channels that introduce both a
burst {\em and} an isolated loss in each window of interest and develop the
associated streaming codes.
All our constructions are based on a layered design and provide significant
improvements over baseline codes in simulations over the Gilbert-Elliott
channel. | 1305.3596v1 |
2013-05-16 | Size-dependent magnetization switching characteristics and spin wave modes of FePt nanostructures | We present a comprehensive investigation of the size-dependent switching
characteristics and spin wave modes of FePt nanoelements. Curved nanomagnets
("caps") are compared to flat disks of identical diameter and volume over a
size range of 100 to 300nm. Quasi-static magnetization reversal analysis using
first-order reversal curves (FORC) shows that spherical caps have lower vortex
nucleation and annihilation fields than the flat disks. As the element diameter
decreases, the reversal mechanism in the caps crosses over sooner to coherent
rotation than in the disks. The magnetization dynamics are studied using
optically induced small angle precession and reveal a strong size dependence
that differs for the two shapes. Flat disks exhibit well-known center and edge
modes at all sizes, but as the diameter of the caps increases from 100 to 300
nm, additional oscillation modes appear in agreement with dynamic micromagnetic
simulations. In addition, we show that the three-dimensional curvature of the
cap causes a much greater sensitivity to the applied field angle which provides
an additional way for controlling the ultrafast response of nanomagnetic
elements. | 1305.3686v1 |
2013-05-28 | Reallocation Problems in Scheduling | In traditional on-line problems, such as scheduling, requests arrive over
time, demanding available resources. As each request arrives, some resources
may have to be irrevocably committed to servicing that request. In many
situations, however, it may be possible or even necessary to reallocate
previously allocated resources in order to satisfy a new request. This
reallocation has a cost. This paper shows how to service the requests while
minimizing the reallocation cost. We focus on the classic problem of scheduling
jobs on a multiprocessor system. Each unit-size job has a time window in which
it can be executed. Jobs are dynamically added and removed from the system. We
provide an algorithm that maintains a valid schedule, as long as a sufficiently
feasible schedule exists. The algorithm reschedules only a total number of
O(min{log^* n, log^* Delta}) jobs for each job that is inserted or deleted from
the system, where n is the number of active jobs and Delta is the size of the
largest window. | 1305.6555v1 |
2013-06-04 | Generalized splines on arbitrary graphs | Let G be a graph whose edges are labeled by ideals of a commutative ring. We
introduce a generalized spline, which is a vertex-labeling of G by elements of
the ring so that the difference between the labels of any two adjacent vertices
lies in the corresponding edge ideal. Generalized splines arise naturally in
combinatorics (em algebraic splines of Billera and others) and in algebraic
topology (certain equivariant cohomology rings, described by
Goresky-Kottwitz-MacPherson and others). The central question of this
manuscript asks when an arbitrary edge-labeled graph has nontrivial generalized
splines. The answer is `always', and we prove the stronger result that
generalized splines contain a free submodule whose rank is the number of
vertices in G. We describe all generalized splines when G is a tree, and give
several ways to describe the ring of generalized splines as an intersection of
generalized splines for simpler subgraphs of G. We also present a new tool
which we call the GKM matrix, an analogue of the incidence matrix of a graph,
and end with open questions. | 1306.0801v2 |
2013-06-04 | Large Chern Number Quantum Anomalous Hall Effect In Thin-film Topological Crystalline Insulators | Quantum anomalous Hall (QAH) insulators are two-dimensional (2D) insulating
states exhibiting properties similar to those of quantum Hall states but
without external magnetic field. They have quantized Hall conductance
$\sigma^H=Ce^2/h$, where integer $C$ is called the Chern number, and represents
the number of gapless edge modes. Recent experiments demonstrated that chromium
doped thin-film (Bi,Sb)$_2$Te$_3$ is a QAH insulator with Chern number
$C=\pm1$. Here we theoretically predict that thin-film topological crystalline
insulators (TCI) can host various QAH phases, when doped by ferromagnetically
ordered dopants. Any Chern number between $\pm4$ can, in principle, be reached
as a result of the interplay between (a) the induced Zeeman field, depending on
the magnetic doping concentration, (b) the structural distortion, either
intrinsic or induced by a piezoelectric material through proximity effect and
(c) the thickness of the thin film. The tunable Chern numbers found in TCI
possess significant potential for ultra-low power information processing
applications. | 1306.0888v2 |
2013-06-07 | Free energy generalization of the Peierls potential in iron | In body-centered cubic (bcc) crystals, ${1}{2}111$ screw dislocations exhibit
high intrinsic lattice friction as a consequence of their non-planar core
structure, which results in a periodic energy landscape known as the Peierls
potential, $U_P$. The main features determining plastic flow, including its
stress and temperature dependences, can be derived directly from this
potential, hence its importance. In this Letter, we use thermodynamic
integration to provide a full thermodynamic extension of $U_P$ for bcc Fe. We
compute the Peierls free energy path as a function of stress and temperature
and show that the critical stress vanishes at 700K, supplying the qualitative
elements that explain plastic behavior in the athermal limit. | 1306.1633v4 |
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